I. system inspection

 

(1) power supply: use a multimeter to measure whether there is electricity behind the main circuit switch (contactor) and control fuse, which will rule out the possibility of open circuit of these components.

 

(2) Rectifier: The rectifier adopts three-phase fully-controlled bridge rectifier circuit, which includes six fast fuses, six thyristors, six pulse transformers and a free-wheeling diode. There is a red indicator on the quick fuse, which shrinks inside the shell when it is normal, and pops up when the quick fuse burns out. Some quick fuse indicators are tight, and when the quick fuse burns out, it will get stuck inside. Therefore, for reliability, you can use a multimeter to measure the quick fuse by turning it on and off to judge whether it burns out.

 

(3) Inverter: The inverter includes four fast thyristors and four pulse transformers, which can be checked according to the above method.

 

(4) Transformer: Each winding of each transformer should be open, and the resistance of the primary side is about tens of ohms, and the secondary side is several ohms. It should be noted that the primary side of the IF voltage transformer is connected in parallel with the load, so its resistance value is zero.

 

(5) Capacitors: The electrothermal capacitors connected in parallel with the load may be broken down, and the capacitors are generally installed on the capacitor rack in groups. When checking, the group of the broken capacitors should be determined first. Disconnect the connection point between bus bar and main bus bar of each group of capacitors, and measure the resistance between two bus bars of each group of capacitors, which should be infinite under normal conditions. After confirming the bad group, disconnect the soft copper skin of each electrothermal capacitor leading to the bus bar, and check one by one to find the breakdown capacitor. Each electrothermal capacitor is composed of four cores, the shell is one pole, and the other pole is led to the end cover through four insulators respectively. Generally, only one core is broken down, and the lead wire on this insulator is tripped. This capacitor can be used continuously, and its capacity is 3/4 of the original one. Another fault of capacitor is oil leakage, which generally does not affect its use, but attention should be paid to fire prevention. The angle steel on which the capacitor is installed is insulated from the capacitor rack. If the insulation breakdown will make the main circuit grounded, measure the resistance between the capacitor shell lead and the capacitor rack to judge the insulation condition of this part.

 

Second, the maintenance of abnormal work after starting

 

Through the above-mentioned inspection, the fault that can't be started at all can be basically eliminated. After startup, it does not work normally, which is generally manifested in the following aspects:

 

(1) the rectifier is out of phase: the fault shows that the sound is abnormal during operation, the maximum output voltage does not rise to the rated value, and the strange sound of the power cabinet becomes louder. at this time, the output voltage can be lowered to about 200V, and the output voltage waveform of the rectifier can be observed with an oscilloscope (the oscilloscope should be placed in the power supply synchronization). under normal conditions, the input voltage waveform has six waveforms per cycle, while under phase failure, two waveforms will be missing. This fault is generally caused by a thyristor of rectifier without trigger pulse or trigger non-conduction. At this time, look at the gate pulses of six rectifier thyristors with oscilloscope first. If any, measure the gate resistance of each thyristor with multimeter 200Ω gear after shutdown, and replace the thyristor which is blocked or has extremely large gate resistance.

 

(2) Fault of induction coil: the induction coil is the load of medium frequency power supply, which is made of square copper tube with wall thickness of 3-5mm. Its common faults are as follows: a. The induction coil leaks water, which may cause the turn-to-turn ignition of the coil, and it must be repaired and welded in time before running. B. The molten steel sticks to the induction coil, and the steel slag is hot and red, which will cause the copper pipe to burn through and must be cleaned in time. C. turn-to-turn short circuit of induction coil, which is particularly easy to happen in small medium-frequency induction furnace, because the furnace is small and deformed by thermal stress during operation, resulting in turn-to-turn short circuit, which is characterized by higher current and higher working frequency than usual.

 

Shandong Rong Tai Induction Technology Co., Ltd. (erongtai.cn) has strong technical research and development capabilities, rich experience in IF design and a relatively complete after-sales service system in various regions of the country, which has won the trust of many famous domestic enterprises.